The over-arching aim of this project is to uncover shared mechanisms that govern developmental and pathologic neuron death and to apply this knowledge to formulate treatments for stroke, trauma and neurodegenerative disorders. The proposed studies will focus on the apoptotic cell cycle pathway studied in depth under this grant that plays a required role in a variety of developmental and pathological neuron death paradigms. In this pathway, the cyclin-dependent kinase-4 (Cdk4) is activated in neurons in response to apoptotic stimuli. This is turn phosphorylates Rb pocket protein family members causing them to dissociate from gene-repressor complexes with the E2F transcription factors. Dissociation of Rb-E2F repressor complexes leads to derepression and induction of Myb transcription factors that bind to and activate the promoter for apoptotic proteins including Bim. Induction of Bim promotes release of mitochondrial proteins and activation of caspases, resulting in neuron death. This proposal focuses on the mechanism by which apoptotic stimuli activate Cdk4 and in particular on the cdc25a phosphatase. During the cell cycle, cdc25a activates Cdk4 by removing an inhibitory phosphate group. We will assess the hypothesis that cdc25a activity is elevated by apoptotic stimuli in neurons and that this is an apical event in triggering the apoptotic cell cycle pathway. Preliminary supportive data include findings that cdc25a protein and transcripts are elevated in neuronal cells by several different apoptotic stimuli, and that knockdown or chemical inhibition of cdc25a is neuroprotective. There are two specific aims. 1). To test, in in vitro models, the hypothesis that cdc25a plays a proximal role in developmental and pathological neuron death and to dissect the up- and downstream elements of the apoptotic cell cycle cascade in which it partakes. This will involve a variety of cell cultue systems and models for developmental (NGF deprivation) and pathological (AD, PD) neuron death. We will explore a) the required role of cdc25a in neuron death in our various models, b) the extent of cdc25a regulation by apoptotic stimuli, c) the death cascade components that lie downstream of cdc25a, and d) the upstream mechanisms by which cdc25a expression and activity are regulated and how these are engaged by apoptotic stimuli. We will be particularly keen to identify feed-forward death loops involving cdc25a. 2). To test the hypothesis that cdc25a is required for developmental neuron death in vivo. Using the paradigm of developmental neuron death in superior cervical ganglia, we will query whether a) cdc25a levels are induced in dying neurons, b) selected downstream cell cycle pathway components are co-elevated in such neurons, c) biologically-tolerated chemical cdc25a inhibitors suppress neuron death in this system and d) whether such death is inhibited by selective genetic ablation of cdc25a in sympathetic ganglia. Current findings indicate that cdc25a is a druggable target whose in vivo inhibition has little side effects. Thus success of the current aims has a clear potential to lead to development of therapies for pathologic neuron death.